RE: Matt printing instead of gloss
Temperature/speed difference. Id guess that the fan doesn't kick in for several layers on the MK4 and it cools more when it does compared to the Mk3. The same difference can be caused by different speeds. Faster speeds mean the filament extrudes faster which means it spends less time in the hotzone, therefore isn't as hot/viscous when extruded. Its basically a combination of factors that see-saw around.
The rougher top on the mk4 could be a bit of over extrusion too. Basically have you tuned your printer and filament ?
RE: Matt printing instead of gloss
I've printed sample test pieces and noticed the same thing when switching from IS Speed profile to IS Structural profile. While using the speed profile, the exterior sides of the print are matte black because the perimeter is being printed at full speed. The structural profile slows down the speed on the perimeter layers, and the result is high gloss. Because it only affects perimeter layers, the overall print speed is reasonably close. I've defaulted to simply use the structural profile for everything for this reason.
-J
RE: Matt printing instead of gloss
Speed is the answer, I believe. Add an MK4 non-IS profile to PrusaSlicer, slice in QUALITY preset, and I'll bet a 6-pack the part will be glossy, just like your MK3 part prints.
This is based on my experience with my MK4 since July. IS was in beta then, so I held off on installing it and used the non-IS firmware. All my prints were glossy, just like on my MINI beforehand. Once I installed the IS firmware, my prints went matte. Prusament and Jessie PLA both went from glossy printing in non-IS QUALITY to matte with IS. The matte is solid matte with the SPEED profile, but the STRUCTURAL profile can still have some matte to it I think instead of the full gloss I was used to.